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  • Tim McGill

Steamy Today & Severe Storm Threat Tonight

Building Heat & Humidity To Fuel Storms

Our streak of 90s stretched to three straight days before ending on Tuesday. Yesterday's high of 87° combined with high humidity still made it feel like 90°. Today's combination of heat and humidity will make it feel much hotter. The forecast is complicated from early morning showers and thunderstorms that will have a cooling effect overall but the atmosphere has time to recover and that means heat and humidity have time to build to high levels once again.

The HRRR Model has our highs climbing into the lower 90s again today but cooler lakeside with highs there in the lower 80s.

Heat index values today could climb to near 100° away from the lakefront. Dew points will be in the 70s meaning oppressive levels of humidity.

So depending on the amount of clearing we see after the morning rain moves out, our highs should be near 90°. Dew points in the lower to middle 70s means our heat index values will be approaching 100°. Our highs drop back to about average on Thursday but even tomorrow's heat index values could reach to near 90°. The dew points then drop significantly to set us up for a more comfortable Friday.

A scattered shower or thunderstorms can't be ruled out today on a lake breeze that will bump into the unstable air as a result of the building heat and humidity. The best chance of rain remains overnight into early Thursday morning. Strong to severe storms are a real possibility. More on this below.

Heat index levels will peak today thanks to the tropical level dew points. Dew points are the preferred measurement of moisture for meteorologists. Dew points stay in the lower to middle 70s through early Thursday before falling into the 50s by Friday. Friday will feel pleasant compared to today and tomorrow.

Wildfire smoke has thinned somewhat but will still be present in our skies today. The HRRR Model vertically integrated smoke forecast continues to keep the thickest wildfire smoke north and west of Illinois again today with more moderate levels here.

The National Blend Of Models has another 90° day today and then a return to more seasonably warmth Thursday. That would bring the streak to five straight days. We fall back below average as a cooler pattern kicks in on Friday. It suggests several days with highs only in the 70s before bouncing back to about average by the middle to end of next week.

The long range temperature probability forecast has now consistently favored us for a relatively cooler start to August. The 6-10 day forecast has outlooked Chicago for below average temperatures overall from August 2nd through August 6th. The longer range 8-14 day forecast favors us for about average temperatures overall from August 4th through August 10th with areas well northwest of the city favored for above average temperatures overall. Average highs are in the lower to middle 80s for the end of July and start to August.

The even longer range Subseasonal Experiment (SubX) temperature anomaly forecasts keeps us around average for the week ending on August 13th with areas just west and south of us expected to be cooler than average. The following week we are forecast to move back above average. The forecast for the week ending on August 20th favors us for above average temperatures overall.

Swim Risk Thursday & Friday

Strong storms early Thursday morning may contribute to conditions favorable for a meteotsunami. Here is the National Weather Service's explanation of a meteotsunami:

Unlike tsunamis triggered by seismic activity, meteotsunamis are driven by air-pressure disturbances often associated with fast-moving weather events, such as severe thunderstorms, squalls, and other storm fronts.

At the very least there could be some choppy waters and a swim risk early Thursday. A cold front later Thursday will shift winds to the north and northeast. Those gusty winds will whip up some big waves (possibly up to 6 feet) later Thursday into Friday along with dangerous rip currents.

Stay tuned and check the forecast if you are headed to the beaches later this week.

Severe Storm Threat Early Thursday

We are now down 1.67" of rain below average for July at O'Hare and nearly 6 inches since the start of spring. Possible strong storms overnight into Thursday morning may bring some relief four our drought stricken northern suburbs.

There was some slightly better news on the drought in our northern suburbs last week. Here is a recap from Friday's post of our drought conditions based on the latest analysis from the US Drought Monitor:

The area of Illinois in a moderate drought (level 1 out of 4) has dropped slightly from 7.32% last week to 7.17% this week. The area of the state in a severe drought (level 2 out of 4) has increased slightly from 2.40% a week ago to 2.31% this week. There is not longer any portion of Illinois experiencing an exceptional drought (level 3 out of 4). Last week just under 1% of the state fell into that category.

Rainfall over a two week period that ended on July 21st shows large portions of our area received very little rainfall during those 14 days. Hardest hit was already parched portions of Lake and McHenry counties where just 10% to 25% of average rainfall fell.

The latest GFS model suggests a scattered shower or thunderstorm possible this afternoon but a better chance for storms overnight into early Thursday morning. We then dry out for Friday and most of Saturday with another chance for some scattered showers and thunderstorms Saturday night into early Sunday morning.

GFS Model Cloud Cover & Precipitation Meteogram For O'Hare

The GFS model total rainfall forecast through Thursday evening has shifted the axis of heavier rain back over our far northern suburbs (compared to yesterday's model run). It squeezes out between one inch to nearly three inches of rain north of the city with nearly an inch in the city itself and then tapering off amounts further south of Chicago.

There is still the possibility some of the storms late tonight into early Thursday could be severe. The Storm Prediction Center has placed most of us in an enhanced risk category for severe storms during this period. The greatest risk from these storms would be damaging winds and hail but tornadoes are also possible. A bowing line of thunderstorms or derecho is a possibility. The last derecho to impact the area swept through last August. The timing and dynamics of tonight's possible storms suggest it won't be as strong or as widespread an event when and if it arrives here.

A marginal risk category is level 1 out of 5 possible risk levels. Slight risk means "an area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity."

A slight risk category is level 2 out of 5 possible risk levels. Slight risk means "an area of organized severe storms, which is not widespread in coverage with varying levels of intensity."

An enhanced risk category is level 3 out of 5 possible risk levels. Slight risk means "an area of greater (relative to Slight risk) severe storm coverage with varying levels of intensity.

The long range precipitation probability forecasts again favor us for a relatively dry start to August and then shifts us to above average rainfall. The latest 6-10 day forecast has the Chicago area favored for below average precipitation overall from August 2nd through August 6th. The longer range 8-14 day forecast has the city and areas just north and northwest of the city favored for average precipitation overall from August 4th through August 10th. Well west and southwest of the city is favored for near normal rainfall during this period.

Here is my 7 day forecast:

Today: Sun & cloud mix, hot & humid, a few sct. t-storms possible High: 90 (83 lakeside)

Thursday: Sct. showers/t-storms mainly early, partly/mostly cloudy Low: 73 High: 85 (cooler lakeside)

Friday: Partly to sunny, cooler, less humid Low: 65 High: 79 (cooler lakeside)

Saturday: Partly to mostly sunny, sct. t-storm possible late Low: 64 High: 84

Sunday: Mostly sunny Low: 65 High: 79 (cooler lakeside)

Monday: Mostly sunny Low: 60 High: 80 (cooler lakeside)

Tuesday: Mostly sunny Low: 62 High: 81 (cooler lakeside)


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